Sitting down or lying down ought to have a relaxing effect. The leg muscles can relax, since they do not have to carry any weight in this position. By contrast, the back muscles could relax, provided that they are not put under stress when the user adopts the desired posture.
When sitting on a seat, only a small part of the back is in actual contact with the back of the seat. The back muscles are continually tensed in order to keep the back in the desired position, so as thereby to compensate for the lack of suitable support provided by the seat back.
By contrast, when sitting in a more upholstered seat, the cushion admittedly assumes the shape of the back. However, the more pressed-in parts of the cushion exert higher pressure on the back than the less pressed-in parts, particularly level with the lumbar region. There is, once again, the same problem that the back muscles are under stress, and the user begins to wriggle about and tries to shift his sitting plane further and further forwards by sinking further and further into the chair until the lower back is once more completely unsupported.
The cause of this restlessness is not only the unequal support which the back receives, but also a need for movement. There is therefore clearly a problem as regards the user's comfort.
Being seated for a long time and standing for a long time both represent a heavy physical load. The body needs to move in order to remain supple.
A good seat back and a good seat therefore ought to assume the shape of the sitter's back and bottom and thereby provide uniform support on all the exposed parts of the body. This characteristic should be maintained when the user moves and adopts different postures.